Teachings of fuel cells, fuel cell stacks, fuel cell stack assemblies, and heat exchanger systems, arrangements and methods are well known to one of ordinary skill in the art, and in particular include WO02/35628, WO03/07582, WO2004/089848, WO2005/078843, WO2006/079800, WO2006/106334, WO2007/085863, WO2007/110587, WO2008/001119, WO2008/003976, WO2008/015461, WO2008/053213, WO2008/104760, WO2008/132493, WO2009/090419, WO2010/020797, and WO2010/061190, which are incorporated herein by reference in their entirety.
Unless the context dictates otherwise, the term “fluid” incorporates both liquids and gases.
Legislation and the general trend of improved environmental responsibility encourages an interest in reducing the emissions produced by the burning or combustion of fuel in all operations. In fuel cell operation in particular, there is legislation which sets maximum limits for emission levels, such as European standard EN 50465:2008 which applies to a fuel cell gas heating appliance when in domestic use. Of particular importance in controlling emissions is the reduction of carbon monoxide (CO) and nitrous oxides (NOx) emissions.
Burner design is of great importance when it comes to controlling combustion emissions. Factors such as the air flow, the mixing of reactants and the position of the flame must all be considered along with the chemical composition of the fuel to be burned. A change in the composition of a fuel combusted in the same burner can result in very different emissions. Therefore, it is often necessary to design a burner for a specific fuel in order to adhere to the required emission limits. Despite this, there are situations where a burner must be fuelled by various fuels, and where combustion stability and emission control is important in each of these modes.
Burners are often used in fuel cell systems to provide thermal energy to raise the temperature of the fuel cell system and its related system parts to operating temperature. A fuel cell system typically includes at least one fuel cell stack.
Where reference is made herein to a fuel cell or fuel cell system then more preferably, the reference is to a solid oxide fuel cell (SOFC) or SOFC system, more preferably to an intermediate temperature solid oxide fuel cell (IT-SOFC) or IT-SOFC system. A fuel cell system will comprise an at least one fuel cell stack, each fuel cell stack comprising at least one fuel cell. More preferably, the fuel cell has, or fuel cells of the fuel cell stack have, an operational temperature range of 450-650 deg C., more preferably 500-610, or 500-615 or 500-620 deg C., and possibly 615 to 620 deg C.
When utilizing solid oxide fuel cells, it is preferable that the burner is fuelled by both a low calorific value (LCV) fuel and a high calorific value (HCV) fuel. It should be noted that these terms are distinct from e.g. “lower calorific value” (also referred to as “LCV”) and “higher calorific value” (also referred to as “HCV”)—all fuels have both a lower calorific value and a higher calorific value. Examples of low calorific value (LCV) fuels are those with a high fraction of H2, CO, and optionally with a low fraction of CH4. The Wobbe index for a LCV fuel is typically between 18 and 35 MJ/m3. Examples of high calorific value (HCV) fuels are those comprising of methane, ethane or propane or any combination therein, the Wobbe index for a HCV fuel is typically between 36 and 85 MJ/m3.
The fuel cell stack uses a hydrogen-rich HCV fuel for the electrochemical reaction. As a result of the electrochemical reaction, the fuel gas changes composition with some of the reactive elements being oxidised, such as hydrogen becoming water vapour and carbon monoxide becoming carbon dioxide. As a result, the off-gases from this process are an LCV fuel. It is therefore clear that a HCV fuel is distinct from an LCV fuel.
The LCV fuel formed from the electrochemical reaction can then be combusted in a burner. However, the combustion of a HCV fuel is typically required to initially heat the fuel cell system (e.g. at start-up) until the fuel cell reaches operating temperature. Thus, at start-up it is necessary to combust an HCV fuel. During steady-state operation of the fuel cell it is necessary to combust a predominantly LCV fuel. During the transition between fuel cell operating point states (i.e. when the electrical power output of the fuel cell is changed), the composition of the fuel to be combusted changes accordingly, and similarly changes during the transition from steady-state to shut-down. To maintain low emissions with the combustion of each of these fuels, different configurations of burner are required: an HCV fuel burner favours a great degree of mixing with an oxidant prior to combustion; whereas an LCV fuel burner favours a low amount of mixing with an oxidant prior to combustion. Furthermore, a greater airflow is preferred for an HCV fuel compared to an LCV fuel. However, due to requirements elsewhere in the system, such as the oxidant flow being used to control the temperature of the fuel cell stack, it is rarely possible to control airflow to the burner solely for combustion control purposes. It is therefore clear that in the situation described, utilizing a burner designed for one of the fuels or for a specific airflow would result in unfavourable combustion for the other fuel.
It is therefore desirable to produce a burner which is able to combust both LCV and HCV fuels either at the same time, or individually, without separating the combustion or utilizing complex systems, whilst maintaining low emissions and coping with the varying airflows and, in particular, a wide ranging air to fuel ratio, lambda.
Prior art devices can also suffer from a lack of flame stability over a wide range of operating conditions, including different lambdas. In addition, it is also desirable to achieve a compact flame in order to reduce product size.
The present invention seeks to improve upon prior art burners. In particular, it seeks to address, overcome or mitigate at least one of the prior art issues.